Gas transport tests

The BGS Transport Properties Research Laboratory (TPRL) is currently involved in a benchmarking exercise for gas transport testing of sealing materials, a technology required for the performance assessment of potential radioactive waste disposal sites.

Shell and NAGRA (Nationale Genossenschaft für die Lagerung Radioaktiver Abfälle) have jointly commissioned a project to engage a number of laboratories across Europe in testing the transport properties of shale material from the Mont Terri Rock laboratory in Switzerland. Tests involve the determination of both gas and hydraulic transport properties of the rock for flow both parallel and perpendicular to bedding, under predefined conditions.

Background

A number of modern geological problems involve the presence of gas in the subsurface environment. One example is the generation of gas by-products from corrosion of canisters used for geological storage of radioactive waste. Interest in such problems is not only in flow through porous media (such as reservoir rocks), but also the low-permeability materials that provide a seal or retard flow. These are generally lithified or unlithified clay-rich materials, the gas transport characteristics of which may control the long-term surety of geological storage systems. Water is ubiquitous in most of these situations and so fluid flow is usually multi-phase. This is a complex system to model. As such, the collection of experimental data is of great importance for many geological storage and/or extraction problems.

In order to determine the gas transport properties of rocks, a number of different experimental methodologies have been developed by different testing laboratories. The timescales allowed for testing vary greatly, as do the pressure regimes and flow geometries. Direct comparison of approaches used between testing laboratories has not previously taken place.

Rock material under test

Figure 1. Laboratory testing of the gas transport properties of rock materials used as sealants in the disposal of radioactive waste.

The rock material used for the cross-laboratory testing is the Opalinus Clay, a mid-Jurassic marine shale (Figure 1). The material is of significant interest to the nuclear disposal industry for its potential suitability as a waste repository host.

Current investigation

Figure 2a. An assembled sample stack before testing, ready to be inserted into the pressure vessel.

We are using a purpose-built gas permeability rig to carry out the tests (Figures 2a and 2b). The sample used (Figure 1) is confined isotropically, whilst permeant (either synthetic pore water or helium gas) is injected into its base. Flow is distributed by two porous stainless-steel discs that can be monitored independently. One is positioned in the centre of the injection surface and the other around its outer circumference. This guard-ring arrangement was developed by the TPRL and has significant advantages: pore-pressure evolution can be monitored, hydraulic anisotropy can be quantified and the degree of sample symmetry can be determined.

The results of this project will improve our understanding of the advantages and disadvantages of the different experimental methodologies used by different laboratories. The investigaton will also provide information on the timescales required for reliable results and the suitability of different geometries for a given problem.